Organic field effect transistor with a photostructured gate dielectric, method for the production and use thereof in organic electronics

ABSTRACT

The invention relates to an organic field effect transistor which is especially characterized by a cross-linked, structured insulating layer ( 4 ) on which the gate electrode ( 5 ) is arranged. The structure of the OFET ensures that the gate electrode ( 5 ) of an OFET can be used as a strip conductor to the source electrode ( 2 ) of the next transistor and can be used in the construction of larger circuits.

CROSS REFERENCE TO RELATED APPLICATIONS

This is the 35 USC 371 national stage of international applicationPCT/DE02/00312 filed on Jan. 29, 2002, which designated the UnitedStates of America

FIELD OF THE INVENTION

The present invention relates to organic field effect transistors,so-called OFETs, with photopatterned gate dielectric as well as a methodfor the production thereof, and the use of said field effect transistorsin organic electronics.

BACKGROUND OF THE INVENTION

Field effect transistors play a central role in all areas ofelectronics. In order to adapt them to suit particular applications, ithas been necessary to make them lighter and more flexible. Thedevelopment of semiconducting and conducting polymers has made itpossible to produce organic field effect transistors, all parts ofwhich, including the semiconductor layer as well as the source, drainand gate electrodes, are fabricated from polymeric materials.

However, in the production of organic field effect transistors aplurality of organic layers have to be patterned one on top of the otherin order to obtain an OFET of normal construction, as shown in FIG. 1.This is possible only to a very limited extent using conventionalphotolithography which is actually used for patterning inorganicmaterials. The operations normally involved in photolithography dissolveor attack the organic layers and therefore make them unusable. Thisoccurs, for example, when a photoresist is spun on, developed andstripped off.

This problem has been solved using an organic field effect transistor asdescribed in Applied Physics Letters 1998, page 108 et seq. Apolyaniline-coated polyimide film is used as the substrate. In thisfirst polyaniline layer, the source and drain electrode are formed byirradiation through a first mask. In this first layer, a semiconductorlayer of polythienylenevinylene (PTV) is also formed, on whichpolyvinylphenol is then crosslinked using hexamethoxymethylmelamineHMMM. This layer is used as the gate dielectric and as an insulator forthe next layer and the interconnects. A further polyaniline layer isfinally formed thereon in which the second layer of interconnects andthe gate electrode is defined by patterning. The vertical interconnectsare produced mechanically by punching pins through the layers.

The above method prevents previously applied layers from being dissolvedor otherwise damaged. However, it has been shown that in particular thelast operation for forming the vertical interconnects (otherwise knownas vias) does not permit the fabrication of complex circuits.

Applied Physics Letters 2000, page 1487 describes how this problem canbe solved by providing low-resistance vias in the field effecttransistor structure by means of photopatterning of photoresistmaterial. To this end, another design of OFET, namely a so-called“bottom gate” structure, is regarded as indispensable. If a “top gate”structure of the same composition were produced, this would result inunacceptable contact resistances in the order of M

.

However, the construction and the operations for patterning this OFETwith bottom gate structure are complex, making it impossible tomanufacture particularly complex circuits economically.

SUMMARY OF THE INVENTION

The object of the present invention was therefore to specify an organicfield effect transistor or a method for the manufacture thereof whichpermits the use of photolithography without attacking or dissolving theorganic layers in all operations as well as making possible aconstruction which provides a simple means of vertical interconnectionbetween conducting tracks at different levels in organic integratedcircuits. The organic field effect transistors must at the same time bemanufacturable cheaply and economically using simple operations.

The subject matter of the present invention is therefore an organicfield effect transistor characterized in that, on a flexible substratethere are disposed, in a first layer, source and drain electrodes aswell as a semiconductor on which, in a second layer, an insulator ispattern-formed and onto which, in a third layer, a gate electrode isdeposited (top gate structure).

The organic field effect transistor according to the invention is lightand extremely flexible, as it is only formed from organic layers whichare mainly patterned by means of photolithography but without usingphotoresist. By means of the patterning of the insulator layer inparticular, the gate electrode of the organic field effect transistoraccording to the invention can simultaneously be used as the conductingtrack to the source electrode of the next transistor.

Advantageous embodiments of the subject matter of the invention willemerge from the sub-claims 1 to 10.

Thus ultrathin glasses, but for cost reasons preferably plastic foils,can be used as the substrate. Polyethylene terephthalate and polyimidefoils are particularly preferred. The substrate must in each case be aslight and flexible as possible. As the thickness of the substratedetermines the actual thickness of the device as a whole—all the otherlayers combined are only some 1000 nm thick—the substrate thickness mustalso be kept as small as possible, normally in the range ofapproximately 0.05 to 0.5 mm.

The source and drain electrodes can consist of wide variety ofmaterials. The type of material will basically be determined by the typeof fabrication preferred. Thus, for example, electrodes of indium tinoxide (ITO) can be produced by photolithography on ITO-coatedsubstrates, the ITO being etched away from the areas not covered byphotoresist. Polyaniline (PANI) electrodes can also be produced eitherby photopatterning or by photolithography on PANI-coated substrates.Equally, electrodes made of conductive polymers can be produced byprinting the conductive polymer directly onto the substrate. Conductivepolymers include, for example, doped polyethylene (PEDOT) or possiblyPANI.

The semiconductor layer consists, for example, of conjugated polymerssuch as polythiophenes, polythienylenevinylenes or polyfluorenederivatives which are solution processable by spin-coating,silk-screening or printing. Also suitable for creating the semiconductorlayer are so-called “small molecules”, i.e. oligomeres such assexithiophene or pentacene, which are evaporated onto the substrate by avacuum technique.

However, an important aspect of the present subject matter of theinvention is the way in which the insulator layer is created. This is acrosslinked insulator which is crosslinked and patterned by means ofphotolithography, i.e. under partial exposure. An insulator material iscrosslinked area by area using a crosslinker under acid catalysis.Suitable insulator materials in the context of the present inventioninclude poly(4-hydroxystyrene) or melamine-formaldehyde resinscontaining hydroxyl groups. The crosslinker is acid-sensitive,specifically hexamethoxymethylmelamine (HMMM). Acid catalysis iseffected by means of a photoinitiator, e.g. diphenyliodoniumtetrafluoroborate or triphenylsulfonium hexafluoroantimonate whichproduce an acid under the effect of light.

The present invention relates to a method for producing an organic fieldeffect transistor wherein a flexible substrate is provided with a sourceand drain electrode as well as a semiconductor and is characterized inthat an insulator is deposited on the semiconductor by applying aninsulator material solution containing an acid-sensitive crosslinker aswell as a photoinitiator, exposing it through a shadow mask covering thesource and drain electrodes, and then baking it, crosslinking beingeffected at the exposed areas and the gate electrode being deposited onthe thus crosslinked and patterned insulator.

BRIEF DESCRIPTION OF THE DRAWING

Details and preferred embodiments of the method according to theinvention will emerge from the sub-claims 12 to 18. The invention willnow be described in further detail with reference to FIGS. 1 to 3 and anexemplary embodiment.

In the accompanying drawings:

FIG. 1 shows the construction of a conventional OFET;

FIG. 2 shows the construction of an OFET according to the invention; and

FIG. 3 shows chemical reactions underlying the production of thecrosslinked, patterned insulator layer.

DETAILED DESCRIPTION OF THE INVENTION

A conventional OFET consists of a substrate 1, source and drainelectrodes 2 and 2′, a semiconductor 3, an insulator 4 and the gateelectrode 5. The conventional OFET requires contact tags 6 for combiningindividual OFETs to form larger circuits.

As shown in FIG. 2, the starting point for producing an OFET accordingto the invention is a similar structure to that of a conventional OFET.In other words, on a substrate 1 there are formed source and drainelectrodes 2 and 2′ as well as a semiconductor layer 3. The source anddrain electrodes 2 and 2′ as well as the semiconductor 3 are in onelayer. On this layer a thin layer of an insulator material such aspoly(4-hydroxystyrene) (PVP) or melamine-formaldehyde resins containinghydroxyl groups is deposited by spin-coating, screen printing or similarprocesses. The solution to be applied contains, in addition to theinsulator material, an acid-sensitive crosslinker such ashexamethoxymethylmelamine (HMMM) and a photoinitiator such asdiphenyliodonium tetrafluoroborate or triphenylsulfoniumhexafluoroantimonate. This layer 4 a is then exposed through a shadowmask 7, preferably with UV light. As a result of exposure, thephotoinitiator produces an acid in accordance with reaction scheme (a)in FIG. 3 which effects the crosslinking between the insulator materialand the crosslinker under the effect of temperature, i.e. in asubsequent baking operation (reaction scheme (b) in FIG. 3). Baking isperformed at relatively low temperatures, approximately between 100° C.and 140° C., preferably at 120° C. This ensures that the unexposed areasremain uncrosslinked, as higher temperatures are required forcrosslinking in the absence of a catalyst. In a final development step,the uncrosslinked insulator is removed by rinsing with a suitablesolvent, such as n-butanol or dioxan. As shown in FIG. 2, a crosslinkedand patterned insulator layer 4 b on which the gate electrode is finallyapplied as described above is thereby produced directly on top of thesemiconductor layer 3.

With the present method, the gate dielectric is therefore produced byphotolithography without using photoresist. This results in an OFETwhose gate electrode can be used simultaneously as the conducting trackto the source electrode of the next transistor. This allows verticalinterconnection between conducting tracks at different levels in organicintegrated circuits.

An exemplary embodiment of this will now be disclosed, specificallyindicating the reaction conditions.

EXEMPLARY EMBODIMENT FOR PRODUCING A GATE DIELECTRIC

5 ml of a 10% solution of poly(4-hydroxystene) in dioxan are mixed with20 mg hexamethoxymethylmelamine and a catalytic trace ofdiphenyliodonium tetrafluoroborate and spin-coated onto a substratealready containing electrodes and semiconductor. The substrate isexposed through a shadow mask and then baked for 30 minutes at 120° C.After cooling, the insulator is removed at the unexposed and thereforeuncrosslinked areas by intensive rinsing with or soaking in n-butanol.The gate electrode is formed thereon.

The OFETs according to the invention are ideally suitable forapplications in the field of organic electronics and in particular forthe production of identification stickers (ID tags), electronicwatermarks, electronic barcodes, electronic toys, electronic tickets,for use in product or piracy protection or anti-theft security.

1. Organic field effect transistor, comprising: a flexible substrate; afirst layer on the substrate; source and drain electrodes and asemiconductor in the first layer; an insulator forming a second layer onthe first layer, the insulator being pattern-formed and formed from aninsulator material crosslinked with a crosslinker in the presence of aphotoinitiator, the insulator pattern being produced by crosslinking theinsulator in the desired pattern by photolithography with uncrosslinkedinsulator outside the pattern and then removing the uncrosslinkedinsulator material; and a gate electrode on the second layer forming athird layer.
 2. Organic field effect transistor according to claim 1,wherein the substrate is an ultrathin glass foil or a plastic foil. 3.Organic field effect transistor according to claim 2, wherein thesubstrate is polyethylene terephthalate or a polyimide foil.
 4. Organicfield effect transistor according to claim 1, wherein the source anddrain electrodes are formed from indium tin oxide (ITO), polyaniline(PANI) and/or conductive polymers.
 5. Organic field effect transistoraccording to one of claim 1, wherein the semiconductor is formed fromconjugated polymers or oligomers.
 6. Organic field effect transistoraccording to claim 1, wherein the insulator material is selected frompoly(4-hydroxystyrene) or from melamine-formaldehyde resins containinghydroxyl groups.
 7. Organic field effect transistor according to claim1, wherein the crosslinker is acid-sensitive, includinghexamethoxymethylmelamine (HMMM).
 8. Organic field effect transistoraccording to claim 1, wherein the photoinitiator is selected fromdiphenyliodonium tetrafluoroborate and triphenylsulfoniumhexafluoroantimonate.
 9. Organic field effect transistor according toclaim 1, wherein the gate electrode is formed from polyaniline, otherconductive polymers or carbon black.
 10. Use of the organic field effecttransistor according to claim 1 in organic electronics.
 11. Use of theorganic field effect transistor according to claim 1 for identificationstickers (ID tags), electronic watermarks, electronic barcodes,electronic toys, electronic tickets, in product or piracy protection oranti-theft security.
 12. Organic field effect transistor according toclaim 1, wherein the insulator pattern is produced by patterning theinsulator with photolithography without using a photoresist.
 13. Methodfar producing an organic field effect transistor comprising: providing aflexible substrate with a source and drain electrode and asemiconductor; forming an insulator from an insulator materialcrosslinked with a crosslinker in the presence of a photoinitiator byapplying an insulator material solution containing an acid-sensitivecrosslinker and a photoinitiator to the semiconductor; exposing theinsulator through a shadow mask covering the source and drainelectrodes; patterning the insulator pattern by crosslinking theinsulator in the desired pattern by photolithography with uncrosslinkedinsulator outside the pattern and then removing the uncrosslinkedinsulator material including baking to effect the crosslinking andpatterning of the insulator at the exposed areas; and depositing a gateelectrode on the thus crosslinked and patterned insulator.
 14. Methodaccording to claim 13, wherein the insulator material is selected frompoly(4-hydroxystyrene) or melamine-formaldehyde resins containinghydroxyl groups.
 15. Method according to claim 13, wherein thecrosslinker is acid-sensitive, including hexamethoxymethylmelamine(HMMM).
 16. Method according to claim 15, wherein the photoinitiatorproduces an acid under the effect of light and is selected specificallyfrom diphenyliodonium tetrafluoroborate and triphenylsulfoniumhexafluoroantimonate.
 17. Method according to claim 13, wherein thesolution containing the insulator material, the crosslinker and thephotoinitiator is applied by spin-coating or silk screen printing. 18.Method according to claim 13, wherein UV light used for exposure. 19.Method according to claim 13, wherein baking is performed at atemperature between 100° C. and 140° C.
 20. Method according to claim19, wherein baking is performed at a temperature of 12° C.